1. Field of the Invention
This invention relates to the removal of lead from spent nitrate based solder strippers, the regeneration of the spent nitrate based solder strippers, and their reuse.
2. Background and Prior Art
A typical printed circuit board uses a copper conductor attached to an insulating support, in a patterned manner. Tin/lead solder is applied onto the copper conductor, typically by electroplating before the copper conductor becomes patterned into the final electrical circuit. The solder film is nominally 0.0003 inches thick which is the standard thickness used in most present day processes. The actual thickness may vary considerably over the board and from board to board, as in any manufacturing process. Once solder is applied to the copper, a thin film of copper-tin alloy forms by solid state diffusion between the copper and solder. This copper-tin alloy film increases in thickness with time, but is typically about 0.000002 to 0.000004 inches thick.
As used in this specification and claims, the word "solder" includes the various low melting point alloys and elements used for electrical soldered connections and for copper etching masks or resists. The majority of such coatings are of various compositions of tin-lead alloys, or are substantially pure tin, but can also include lead-free alloys, pure lead coatings, and alloys containing silver, bismuth, cadmium, indium, and other metals. Such films are produced by various methods, including electroplating, chemical deposition and immersion in a melt. This patent invention is directed specifically to solders containing more than about 5% of lead as one of the components.
When applied as a bulk film, the solder may be used as a resist over the copper during etching of the copper in the production of a solder mask over bare copper boards. During the manufacturing process the solder film is applied in an image-wise fashion to those areas which will be the final copper conductor. The uncoated copper is then etched from the insulating and supporting base by means of selective chemical etchants for the copper, such as ammoniacal cuptic chloride solution. The solder film is later stripped from the copper substrate to allow for final fabrication, assembly, and testing. In other applications, solder is used to join two or more electrical conductors together. This solder may be removed by solder strippers for more effective rework, during salvage of board components, and during treatment of scrap and recycle boards to remove lead to reduce environmental liabilities.
Two types of compositions have been generally used in the past for solder stripping. The most widely used prior compositions were based on mixtures of an acidic solution of hydrogen peroxide and fluoride, as described in U.S. Pat. No. 4,673,521 issued Jun. 16, 1987.
Acidic fluoride with peroxide solutions break down during both use and storage. They attack copper easily, leading to large amounts of soluble copper in addition to dissolved lead and tin. The fluoride content of the solutions leads to rapid attack of the fiberglass used as a component of the insulating substrate. This gives an additional toxic material, fluorosilicic acid, for waste treatment. Fluoride is both extremely toxic to the operators and difficult to effectively waste treat.
More recently, formulations based on methansulfonic acid, U.S. Pat. Nos. 4,921,571 and 4,957,653 issued Sep. 18, 1990; or nitric acid solution described under various U.S. patents including U.S. Pat. No. 5,219,484 issued Jun. 15, 1993 and U.S. Pat. No. 5,244,539 issued Sep. 14, 1993; both of which types contain ferric ion have become widely commercially used. This patent application specifically relates to the treatment of acidic solder strippers containing ferric nitrate.
Nitric acid based solutions eliminate many of the problems of fluoride based strippers. The earliest nitric acid strippers consisted of two sequential solutions, consisting of a nitric acid solution to dissolve the solder, followed by a second acidic solution containing ferric ions or other materials to remove the tin-copper alloy. The second solution may contain ferric chloride, ammonium persulfate, a mixture of hydrogen peroxide and sulfuric acid, or a conventional acidic hydrogen peroxide-fluoride mixture.
Ferric nitrate containing compositions for stripping lead based, lead-tin, and similar solders from printed circuit boards or solder masks are described in the following recently issued United States patents. The composition of U.S. Pat. No. 4,713,144 issued Dec. 15, 1987 is a combination of nitric, ferric and sulfamic acid which strips quickly but generates a large amount of tin rich sludge. Formulations containing organic acids such as those described in U.S. Pat. No. 5,219,484 issued Jun. 15, 1993, have been used to attempt to solve the sludge problem but have not been entirely satisfactory in that they only delayed the formation of sludge.
The composition described in U.S. Pat. No. 5,244,539 is a combination of nitric, ferric, and ammonium ions in combination with urea as a nitric acid fume suppressant and organic triazoles as copper antitarnish agents. This gives a low sludging solution but the urea reacts with nitric acid to give the heat-sensitive explosive urea nitrate which precipitates from the solution.
The basic composition and method for single bath and spray stripping is now well described in prior art patents such as those noted above. Commercialization of this process is dependent on meeting most of the following customer expectations: complete stripping in a reasonable minimum time, low attack rate on the exposed copper, a bright surface on the exposed copper which does not rapidly tarnish, long solution stripping life, little toxic fume evolution, and little or no sludge formation.
While effective ferric nitrate acid based strippers are now readily available, there is no effective waste treatment method which leads to the recycle and reuse of such solder strippers. The usual procedure is to treat the spent stripper with caustic to neutralize the solution and give a bulk precipitate of lead, copper, tin, and iron salts; then remove any remaining soluble lead or copper by use of diethyldithiocarbamate, sulfide, or other waste treatment compound. This method serves to waste treat the solder stripper only. None of these patents describes the regeneration of such compositions after they have been used, except U.S. Pat. No. 4,944,851 which uses electrolytic regeneration. U.S. Pat. No. 4,944,851 describes electrolytic methods for reclaim of solder only from used solder stripper solutions based on methylsulfonic acid. There are many disadvantages, including the need for a sealed three compartment plate-out cell, ion selective membrane separators, and electrolytic rectifiers. Nitrate must be excluded as it interferes with formation of a coherent film and the reduction of nitrate reduces the cathode efficiency. There is no available general method which can selectively remove lead from all ferric nitrate based solder strippers and allow the solder stripper to be effectively reused.
It is a principal object of the present invention to provide a new method for selective removal of lead from spent ferric nitrate based solder stripper, the addition of regeneration chemicals, and the reuse of the regenerated ferric nitrate based solder stripper one or more times. These and other objects, advantages, features and results will more fully appear in the course of the following description.